A typical cellular telecommunication system cell is organized about a cell station (often called a base station) equipped with multiplexing equipment for accepting incoming telephone land lines and multiplexing the incoming voice lines onto a radio frequency (RF) carrier that is broadcast by an antenna system over the region that the cell is designated to cover. Individual subscriber stations are each equipped to receive the broadcast modulated carrier and to demultiplex the specific channel that carries the data that is intended for it to receive. Often, two way conversation is supported by full duplex operation on each traffic channel. Hence, the label traffic channel will be used in the context of full duplex operation. An uplink traffic channel is the portion of a traffic channel carrying data from a subscriber station to a cell station, and a downlink traffic channel is the portion of a traffic channel carrying data from a cell station to a subscriber station.
In a typical wireless communication system, an assigned RF bandwidth of frequencies is simultaneously shared by multiple subscribers using various multiple access techniques. Most commonly, frequency division multiple access (FDMA) and time division multiple access (TDMA) techniques are used for sharing the assigned bandwidth amongst a number of subscribers. FDMA subdivides the available bandwidth into a number of sub-bands. Each sub-band accommodates a carrier that is modulated by a subscriber's data In TDMA, multiplexing of multiple subscribers is accomplished by time-sharing in which each subscriber involved in a connection is allocated a periodic time-slot for transmission of its data as a packet. Lately, code division multiple access (CDMA) methods have been introduced for accommodating multiple subscribers on a single carrier (or subcarrier) in which each subscriber is assigned a code waveform that is used to modulate the carrier for each bit of digital data. Each active subscriber having an assigned coded waveform taken from a set of orthogonal waveforms allows the system to separate (demodulate) the individual subscriber transmissions.
Cellular communication systems may also include the use of the more recently introduced spatial division multiple access (SDMA) technique that provides increased subscriber system capacity in systems that use FDMA, TDMA, and/or CDMA methods without any increase in allocated RF bandwidth (Roy, III et al., U.S. Pat. No. 5,515,378) through the use of cell station antenna arrays. SDMA exploits the spatial distribution of subscribers in order to increase the usable system capacity. Because subscribers tend to be distributed over a cell area, each subscriber will tend to have a unique spatial signature characterizing how the cell station antenna array receives signals from, and transmits signals to, the subscriber cell station antenna array. Consequently, the cell station, by determining the spatial signature, has the potential to control the radiation pattern of the cell station antenna array so that the effective antenna gain in the direction, or vicinity, of each active subscriber is optimized, i.e. a lobe maximum is created for each direction, or vicinity, and each lobe is sufficiently narrow so that each active subscriber can be isolated at the cell station for both transmission and reception. The necessary data (referred to as the spatial signature of a subscriber) for implementing SDMA is obtained empirically from the transmissions received by the cell station from each active subscriber. It should be noted that non-spatial multiplexing (e.g., FDMA, TDMA, and CDMA), when used in combination with controllable antenna array patterns that are controlled by using spatial signatures, is referred to as SDMA in the context of this invention. (In practice, spatial signatures and antenna arrays can be used in a non-spatial-division-multiple-access system configuration for enhancing communications between the cell station and subscribers by use of spatial signal processing techniques. In these cases, the label SDMA will still be used in the context of the description of the invention that follows.)
A practical system may consist of one or any combination of CDMA, FDMA, and TDMA techniques. For, example, a combination of FDMA and TDMA techniques may be used in which a set of sub-bands are each further divided into time slots.
If the spatial signatures are used, the effective radiation patterns of the antenna array can allow more than one subscriber to use a given packet time-slot. For example, if the effective radiation pattern of a first subscriber results in a relatively low energy "null" in the vicinity of a second subscriber sharing a packet time allocation, and the second subscriber's spatial signature results in a null in the vicinity of the first subscriber, the simultaneous RF packet transmissions will not cause interference upon reception at the two subscriber stations. Also, transmissions from the two subscribers to the cell station will be separable at the cell station. Under these ideal conditions, the spatial signatures are said to represent an "orthogonal" implementation.
The concept of orthogonality also applies to FDMA and TDMA systems. If each subcarrier in a FDMA is completely isolated so that the modulating data in any of the subcarriers does not affect the data modulating any other subcarrier, then all subcarrier channels are orthogonal to one another. Similarly, in a TDMA system, if each subscriber channel allocated packet data has no effect on any other active channel, the channels are orthogonal to each other.
Orthogonality can be destroyed in each of these multiple access systems. For example, intrachannel interference can result in FDMA systems from carrier frequency offsets and imperfect filters; in TDMA systems from clocking errors and instabilities; in CDMA systems from synchronization inaccuracies or RF multipath; and in SDMA systems from antenna pattern leakage caused by finite dimensional antenna arrays. In practical systems that may involve hundreds of subscriber stations, full orthogonality between every subscriber station can not be insured because of the complexity and cost that such a requirement would place on the system design. Also, an underlying motivation for the use of cellular systems is the re-use of the same RF spectrum in cell areas assigned to different locations. This frequency re-use principle introduces inter-cell interference which can severely degrade communication quality if not carefully controlled, and ultimately limit system capacity.
Because of the fragile nature of orthogonality and because of the interference introduced by cellular frequency re-use, all cellular multiple access communication systems need a method for channel assignment that minimizes the adverse effects caused by less than perfect orthogonality between channels when a new subscriber connection is added to system.
Also, because a basic tool for minimizing interference is the management of radiated power, it is important that minimum radiated power be used by both subscriber and cell stations so as to minimize any interference that may result in any practical multiple access communication system. Also, because any practical implementation must recognize that RF transmissions in one cell can create interference in another neighboring cell, because fall orthogonality between neighboring cellular systems is generally impractical, and because direct real-time communications between neighboring cell stations may not be feasible, a further requirement of a cellular system is that means be provided for minimizing adverse effects from any interference that results from operating one cellular system in the neighborhood of another. Because real-time intercellular communications between cell stations may not exist or may not be feasible, the minimization of the adverse effects of intercellular interference must be considered even in the absence of direct real-time communication between cell stations.
A particular example of an existing protocol for establishing a connection in a cellular communication system between a subscriber station and the cell station (FIG. 1) is that which is used in the "Personal Handy Phone System" described in the Association of Radio Industries and Businesses (ARIB) Preliminary Standard, Version 2, RCR STD-28, approved by the Standard Assembly Meeting of December, 1995.
The system described by ARIB Preliminary Std., Version 2, is a digital wireless personal communication system for communicating between multiple, geographically dispersed, personal handy phone stations (PSs) and a cell station (CS) by RF carrier, for serving the PSs in a given cell and for interfacing to standard telecommunications circuit equipment. The system includes:
(a) 77 RF carriers, spaced 300 kHz apart, over a public system RF band at 1,895-1,918 MHz; PA1 (b) quadrature phase shift keying (QPSK) modulation using multiples of .pi./4 radians phase shift each symbol period; PA1 (c) TDMA-TDD (time division multiple access, time division duplex) RF access for 4 duplex channels per RF carrier; PA1 (d) 384 kbits/s signal transmission rate; and PA1 (e) 5 ms frame length with 120 symbols (including guard bits) per slot. PA1 (1) the CS paging on the paging channel (PCH) of the selected PS to which an incoming connection is desired; PA1 (2) the selected PS responding on the signaling control channel (SCCH) by sending a link channel establishment request; PA1 (3) the CS responding to the PS request by selecting a traffic channel (TCH) and sending the selected TCH as a link channel (LCH) assignment to the PS on the SCCH; PA1 (4) the selected PS switching to the assigned LCH and transmitting a sequence of synchronization (SYNC) burst signals followed by a sequence of idle traffic bursts; and PA1 (5) upon successful detection of a synchronization signal, the CS responds by transmitting a sequence of SYNC bursts on the LCH followed by a sequence of idle traffic bursts and then proceeding to establish a connection with the incoming call to the CS, invoking any additional optional signaling that may be required (e.g. encryption and user authentication). PA1 (1) the PS sending a link channel establishment request on the signaling control channel (SCCH); PA1 (2) the CS responding to the PS request by selecting a traffic channel (TCH) and sending the selected TCH as a link channel (LCH) assignment to the PS on the SCCH; PA1 (3) the PS switching to the assigned LCH and transmitting a sequence of synchronization (SYNC) burst signals followed by a sequence of idle traffic bursts; and PA1 (4) upon successful detection of the synchronization signal, the CS responds by transmitting a sequence of SYNC bursts on the LCH followed by a sequence of idle traffic bursts and then proceeding to establish a connection with the incoming call to the CS, and invoking any additional optional protocols that may be required (e.g. encryption and user authentication). PA1 (1) the CS paging on the paging channel (PCH) of the selected PS to which a downlink connection is desired; PA1 (2) the selected PS responding on the signaling control channel (SCCH) by sending a link channel establishment request; PA1 (3) the CS selecting a traffic channel (TCH) as a tentative link channel (LCH), responding to the PS request by sending the tentative LCH assignment to the PS on the SCCH; PA1 (4) the selected PS switching to the assigned LCH and repeatedly transmitting a synchronization (SYNC) burst signal using a prescribed initial power level for the initial transmitting of the SYNC burst and incrementing the power level at each repeated SYNC burst transmission until a SYNC burst is successfully received from the CS and then transmitting a sequence of idle traffic bursts, the last power level used for SYNC burst being used for all subsequent transmissions to the CS during the ensuing connection; PA1 (5) upon receiving an adequate quality SYNC burst transmitted by the PS (PS SYNC burst), the CS computing the PS transmitter power required based on the time delay between the CS sending the tentative link channel assignment and successfully receiving the adequate quality PS SYNC burst, the CS responding by transmitting a SYNC burst on the LCH followed by a sequence of idle traffic bursts using the computed PS transmitter power as a guide for the CS transmitter power required to adequately communicate with the PS, and then proceeding to establish a connection with the incoming call to the CS, after invoking any additional optional protocols that may be required (e.g. encryption and user authentication).
The control sequence for setting-up and establishing an incoming call to a PS from the CS is shown in FIG. 2. This incoming call connection establishment phase includes:
The PCH is a one-way downlink point-to-multipoint channel on which the CS transmits identical information to all PSs in the paging area. The SCCH is a bidirectional point-to-point channel that transmits information needed for call connection between the CS and a PS. The TCH is a point-to-point bidirectional channel for transmitting user (subscriber) information.
The problem with the above existing procedure is that it does not provide for setting transmitter power levels that are adequate for each connection and it does not address the impact of the interference that would result from the new connection on existing subscribers.
FIG. 3 shows the control sequence for establishing an uplink connection initiated by a PS desiring to connect to the CS for establishing a connection. The steps include;
As in the previous procedure for establishing a downlink connection, the procedure for establishing an up link connection suffers from the same deficiencies: no method for establishing the transmitter power levels required for adequate communications, and no method for evaluating the impact of the interference that is generated by establishing the new connection on existing users.
Control procedures used to establish connections with a PS use common and individually assigned time slots. FIG. 4 shows the time slot assignments used for sending and receiving in the TDMA-TDD system. The time structure of each TDD carrier is organized into 5 ms frames that are divided into 8 segments each. Each segment supports a one-way voice channel of 32 kbits/s (excluding overhead). FIG. 4 is an example that shows the activity on a common 5 ms frame when two PSs (PS(1) and PS(2), and respectively assigned to slots 2 and 4) are communicating with the CS. Normally, the first four slots are assigned for transmission by the CS and, hence, for reception by the PS to which the transmission is directed. The last four slots are used for CS reception and PS transmission. The slots labeled I indicate idle slots. The slot label T(.) indicates transmission during that slot, while R(.) indicates reception. Thus, the frame labeled (a) shows the CS activity: in slots 2 and 4, the CS is respectively transmitting to PS(1) and PS(2), while in slots 6 and 8, the CS is listening to PS(1) and PS(2) respectively. At PS(1), slot 2 is used for reception of the corresponding CS slot transmission, while slot 6 is used for transmission to the CS. Similarly, frame (c) shows the reception and transmission activity of PS(2). Thus, each frame can handle a maximum of 4 bidirectional communications between the CS and 4 PSs.